1. Field of the Invention
This invention relates to a sub-atmospheric pressure gas delivery system for supplying gas to a gas-consuming process facility such as a semiconductor manufacturing tool, wherein the system utilizes sub-atmospheric gas supply vessels and is auto-switchable in character from an empty vessel to a full one without pressure spikes or flow perturbations.
2. Description of the Related Art
Modem semiconductor fabrication plant (fab) operations demand continuous delivery of process chemicals to maximize tool and fab uptime and utilization. For the continuous delivery of compressed and compressed liquefied process gases from supply vessels, conventional delivery systems typically incorporate an automated switching function. This auto-switch function, as it is known, allows users to swap supply cylinders without necessitating the shut-down of critical processes in the fab.
The method of auto-switch for compressed and compressed, liquefied gases is quite simple. Both compressed and compressed liquefied gases are delivered at pressures above atmospheric pressure in the delivery line to the process tool.
For compressed gases, most delivery systems use regulators to step down high cylinder pressures, e.g., 800-1200 psig, to working pressures of 20-100 psig. Compressed gas cylinders are generally changed out and removed from service when the cylinder pressures reach between 100-200 psig. This typically means that the delivery line pressure, i.e., the pressure downstream of the delivery regulator, is not affected by the gradual loss in cylinder pressure, provided that the cylinder pressure remains above the delivery line pressure setpoint--which is the intent of the designs of virtually all compressed gas delivery systems.
For compressed liquefied gases, similar designs have been made. The most obvious difference can be found for lower pressure compressed liquefied gases such as BCl.sub.3 and WF.sub.6, which have cylinder pressures of 4.4 and 2.4 psig, respectively. These systems do not necessarily rely on regulators for maintaining constant delivery line pressures. Instead, most of these compressed liquefied gas delivery systems rely on cylinder and delivery line heating to maintain constant pressures in the delivery line. Such constant delivery line pressures may be positive or negative. By way of illustration, the constant pressure in the delivery line for dispensing of WF.sub.6 may be on the order of about 12 psia.
In either case, for compressed gases or compressed liquefied gases, the fundamental design of the dispensing system provides consistent pressure in the process delivery line. This design capability makes auto-switching a matter of toggling the isolation valve of identical delivery manifolds. The pressure in the delivery line is not drastically different from one compressed cylinder to another when the auto-switch mechanism is enabled. As a result, the downstream process is not exposed to any pressure fluctuations during auto-switch of the dispensing system.
Enabling an auto-switch function for sub-atmospheric gas sources is difficult with current process gas delivery systems for compressed and compressed liquefied gases. The main reason for this is that conventional compressed and compressed liquefied gas systems are designed for "consistent" or "stable" pressure operation. These systems neither provide the capability to deliver consistent sub-atmospheric pressure gas nor the functionality to switch automatically between two sub-atmospheric cylinders that may be at different absolute pressures. Based on the current state of the art, switching between "empty" and "full" sub-atmospheric cylinders would result in pressure spikes throughout the tool and associated gas flow lines. Such pressure spikes can result in particle generation and mass flow inconsistency and instability, both of which can negatively affect process performance and capability.
Unlike compressed gas delivery systems, the current design of single cylinder sub-atmospheric delivery systems does not include the use of mechanical regulators such as diaphragm regulators. There are various reasons for this. One reason is that conventional mechanical regulators induce unwanted pressure drop. Further, mechanical regulators are not available that are capable of efficient operation at very low subatmospheric pressures. Indeed, most conventional mechanical regulators cannot be configured to operate at pressures below about 300 Torr. Without the use of regulators, the absolute pressure of sub-atmospheric gas in the delivery line from a sub-atmospheric gas delivery system is nearly equal to that of the sub-atmospheric gas cylinder. As such, the pressure in the delivery line will decrease as gas is extracted and transported from the sub-atmospheric cylinder. Sub-atmospheric cylinder pressures range from 10-700 Torr, depending on the fill state of the cylinder. The pressure of a "fill" sub-atmospheric cylinder is 650-700 Torr. "Empty" sub-atmospheric cylinders have pressures of 10-30 Torr. The auto-switch between these two states is not recommended for the reasons mentioned in the previous paragraph. A control mechanism is needed that ensures a gradual transition from the pressure conditions of empty sub-atmospheric cylinders to those of full sub-atmospheric cylinders.
Sub-atmospheric pressure gas supply sources of the aforementioned type include the sorbent-based gas storage and dispensing systems disclosed in U.S. Pat. No. 5,518,528 issued May 21, 1996 in the names of Glenn M. Tom and James V. McManus. The gas storage and dispensing system of the Tom et al. patent comprises an adsorption-desorption apparatus, for storage and dispensing of a gas, e.g., a hydride gas, halide gas, organometallic Group V compound, etc. The sorbent-based gas storage and dispensing system of the Tom et al. patent reduces the pressure of stored sorbate gases by reversibly adsorbing them onto a carrier sorbent medium such as a zeolite or activated carbon material, for subsequent dispensing by pressure differential-mediated and/or thermal differential-mediated desorption, optionally with flow of a carrier gas through the gas storage and dispensing vessel providing a concentration differential mediating desorption of the stored gas from the sorbent in the sub-atmospheric pressure supply vessel.
In a typical arrangement of a sorbent-based gas storage and dispensing system of the type described in the Tom et al. patent, a dispensing assembly is coupled in gas flow communication with the storage and dispensing vessel containing the solid-phase sorbent having gas sorbed thereon. The gas dispensing assembly in such system is constructed and arranged to provide, exteriorly of the storage and dispensing vessel, a pressure below the interior vessel pressure, to effect desorption of sorbate gas from the solid-phase physical sorbent medium, and gas flow of desorbed gas through the dispensing assembly.
The storage and dispensing vessel of the Tom et al. patent thus embodies a substantial improvement over high pressure gas cylinders, particularly where hazardous gases are involved. Conventional high pressure gas cylinders are susceptible to leakage from damaged or malfunctioning regulator assemblies, as well as to rupture or other unwanted bulk release of gas from the cylinder if internal decomposition of the gas leads to rapid increasing interior gas pressure in the cylinder.
It is accordingly an object of the present invention to provide an auto-switching sub-atmospheric pressure gas delivery system.
It is another object of the invention to provide an auto-switching sub-atmospheric pressure gas delivery system for sub-atmospheric pressure sorbent-based gas storage and dispensing vessels.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.